The present invention relates generally to a head gasket for an air cooled overhead valve internal combustion engine.
Air cooled overhead valve internal combustion engines include a cylinder portion and a cylinder head which are bolted together. The cylinder portion includes a cylinder bore opening to an end face and a push rod cavity spaced from and generally parallel to the cylinder bore and opening to the end face of the cylinder portion. The cylinder head includes a combustion chamber forming the top of the cylinder bore, an intake valve and an exhaust valve communicating the combustion chamber with intake and exhaust ports, respectively, valve springs, rocker arms, and a push rod cavity. The combustion chamber and push rod cavity of the cylinder head open to an end face in alignment with their counterparts in the cylinder portion. A gasket is provided between the end faces of the cylinder portion and the cylinder head to seal the interface therebetween.
Cylinder head gaskets for an internal combustion engine as described above have conventionally been constructed of a compressible yet resilient material. When the cylinder head is bolted onto the cylinder portion, the head gasket is compressed therebetween with the opposite surfaces of the head gasket intimately engaging the end faces of the cylinder portion and cylinder head to fill any minor surface irregularities therein and effect a high pressure seal between the cylinder portion and cylinder head.
The sealing effectiveness of the conventional head gasket over time depends in large part on its ability to remain resilient in the presence of high temperatures and repeated thermal cycling. As the engine is operated, the cylinder portion, cylinder head, connecting bolts and other metal members of the engine adjacent the head gasket undergo temperature induced expansion and contraction. Consequently, the compression load on the head gasket induced by the bolts connecting the cylinder head to the cylinder portion varies with the temperature of the engine and thus the head gasket must expand resiliently to maintain a high pressure seal.
It has been appreciated that a head gasket made of metal, particularly a relatively soft metal such as aluminum or copper or alloys thereof, provides superior sealing characteristics between the cylinder portion and cylinder head of an internal combustion engine as described above. The superior seal of a metal gasket is obtained due to the malleability of the metal which permits the gasket to readily conform to minor surface irregularities in the end faces of the cylinder portion and cylinder head. Unfortunately, metal head gaskets tend to develop leaks over time as the engine undergoes repeated thermal cycling. Some improvement in the long term sealing capabilities of metal head gaskets has been obtained by the use of local deformations in the gasket to increase the load per unit area on the gasket in such localized areas. This measure by itself, however, has not been wholly satisfactory in curing the problem of metal head gaskets leaking after repeated thermal cycling.
A problem with non-metallic head gaskets is that they function as an insulator between the head and cylinder, thereby blocking effective heat transfer between the hotter head surface and head gasket face and the cooler cylinder gasket surface and cylinder. In the past, metallic gaskets have been used between the head and cylinder surfaces, and in some cases the gaskets have extended outwardly into the cooling fin area of the head and cylinder to thereby be exposed to the cooling air stream of the air cooled engine. However, such prior art gaskets, which were rigidly clamped in place by means of head bolts, leaked after a period of time, and therefore proved to be unfeasible.
It would be desirable to provide means permitting a metal head gasket with its superior initial sealing capability to be used between the cylinder portion and cylinder head of an internal combustion engine while avoiding the problem of gasket leakage which arises due to the relative lack of resilience of a metal head gasket.
It is a characteristic of air cooled overhead valve internal combustion engines that the rocker arm of the valve mechanism is located at a relatively great distance from the cam shaft, with a relatively long push rod connecting the rocker arm to the valve lifter which engages the cam of the cam shaft. Because of the spherical bearing surface of a stamped rocker arm, some means are required for stabilizing the push rod against lateral movement which could cause the rocker arm to pivot about its rocker arm stud, leading to misalignment of the valve mechanism. Prevention of lateral displacement of the push rod has previously been provided by push rod guides in a push rod plate disposed adjacent the rocker arm stud.